1. Statement of the Technical Field
The inventive arrangements relate to radio frequency modulation techniques, and more particularly to an improvement in frequency shift keying (FSK).
2. Description of the Related Art
Frequency-shift keying (FSK) is a conventional method for modulating an RF signal, FSK is a special type of frequency modulation which uses a baseband modulation signal to selectively vary a transmitted signal between two or more predetermined instantaneous frequency values. One common arrangement uses just two predetermined instantaneous frequencies. In such systems, it is common to refer to one frequency as the mark frequency and to a second frequency as the space frequency.
In some conventional types of FSK, there can be a phase discontinuity that occurs when the FSK modulated RF signal transitions from a sinusoidal waveform with one frequency value to a sinusoidal waveform with a second frequency value. Such a phase discontinuity can have several negative aspects. For example, such phase discontinuities can cause amplifier distortion and can result in the presence of wideband components in the modulated RF signal. In order to avoid such problems, it is known in the art that phase discontinuity in an FSK signal can be eliminated by using a technique known as continuous-phase frequency-shift keying (CPFSK). As the name implies, an RF signal that is modulated using CPFSK excludes any abrupt change in the phase angle.
Notwithstanding the continuous phase aspect of CPFSK signals, the frequency of the modulated signal can vary instantaneously when the modulated RF signal transitions from a sinusoidal waveform with one frequency value to a sinusoidal waveform with a second frequency value. Such an instantaneous frequency discontinuity can have several negative aspects. For example, such frequency discontinuities can increase the bandwidth of the transmitted signal, but not to the extent that the phase discontinuities would in the case of a non CPFSK modulator.
When FSK is received in a multi-path rich environment, a guard period is usually employed where the initial N samples of the transmitted signal are discarded in the reception process. The purpose in doing so is to avoid the use of samples that may have been corrupted by multi-path transmission of the previous symbol which may have been sent at a different frequency. The actual guard period can vary in different system designs, depending on various design considerations. For example a guard period may consist of 25% of the total time during which a particular symbol is transmitted. The guard period will typically begin when the transmitted signal transitions from one frequency to a second frequency.